I got a similar message when moving from a 2.2 container to a 2.3 web container. In my case, the problem was with the following lines:

Apparently the 2.2 container would tolerate WEB-INF/struts-config.xml, but the 2.3 container needed /WEB-INF/struts-config.xml. This may not be the same problem you're encountering, but it's worth mentioning.